1. Field of the Invention
The invention relates to tire testing apparatus of the type which measures the vertical load and traction forces in longitudinal and lateral directions with respect to the direction of tire travel on pneumatic tires while in contact with pavement. More specifically, the invention is concerned with tire testing machines of the aforementioned type which use force transducers which include strain gauges for sensing the aforementioned forces.
2. Description of the Prior Art
One type of prior art tire testing machine is described in a publication authored by A. D. Cortese and C. S. Rockafellow entitled "General Motors Proving Ground Tire-Cornering Test Vehicle" which was prepared for presentation at the S.A.E. Automotive Engineering Congress, Detroit, Michigan, Jan. 11-15, 1971. Their solution for measuring tire traction forces consisted of using a towed trailer having a test tire, whose traction forces were being measured, suspended from a frame attached to the trailer by means of a suspension system having a rigid axle housing, four trailing arms and coil-springs similar to those used in conventional automotive rear suspension systems. A triaxial force transducer was utilized to measure the traction forces. Their apparatus utilized a conventional disc brake assembly for generating slip between the tire surface and the pavement. Because this apparatus did not utilize a driven axle to produce slip between the tire and pavement surface, it was impossible to sustain steady state slip rates above approximately 15%. It also was impossible, with this apparatus, to produce driven slip rates which approximate those conditions during which a tire is rotating faster than the rate of speed of the trailer to which the tire testing apparatus was secured.
A second type of prior art apparatus was described in a paper presented to the Society of Automotive Engineers at the National Automotive Engineering Congress, Detroit, Michigan, during Jan. 8-12, 1973. This paper was entitled "Tire Cornering Traction Test Methods" and was authored by C. Beauregard and R. G. McNall. The apparatus, like the one described in the publication discussed supra, is secured to the frame of a towed trailer. Upper and lower arms suspended a tire and hub assembly from the frame of the trailer. Spherical bearings attached the ends of the upper and lower arms to the hub assembly and to the frame. Slip between the tire and road surface was produced by the application of a braking force applied by a disc brake rotor attached to the rotating part of the hub assembly to which the tire was attached. A biaxial force transducer was used to measure the traction force in the plane of the road surface parallel to the direction of tire travel and the load applied to the tire in a direction perpendicular to the road surface. A second independent force transducer was used to measure the traction force in the plane of the road surface perpendicular to the direction of tire travel. This device suffers from the same disadvantages of the device described supra. Additionally, this apparatus required the use of two independent force transducers.
A third prior art pneumatic tire testing apparatus was described in a paper entitled "Measurement of Tire Shear Forces" authored by Howard Dugoff and B. J. Brown for the Automotive Engineering Congress held at Detroit, Michigan from Jan. 12-16, 1970. This apparatus and a similar apparatus developed at the National Aeronautics and Space Administration used a truck without a separate trailer having a wheel assembly suspended from the truck frame to test a tire. The suspension systems of these two apparatus utilized a ball spline linear bearing to permit vertical movement of the tire with respect to the truck frame and the surface on which it was being tested. The vertical load applied to the test tire was applied by a variable volume gas cylinder. The disadvantage of using a ball spline linear bearing for permitting vertical motion of the wheel assembly is that it has a greater tendency to bind than a wheel assembly suspended by upper and lower arms pivoted upon spherical bearings. Vertical movements of the truck frame produce fluctuation in the volume of the load gas cylinder which changes the load applied to the tire. Unlike the apparatus described in the preceding publications, the test tire in each of these apparatus has slip between it and the road surface produced by means of a motor-driven live axle. The forces sustained by the tire during testing were sensed by a combination of a biaxial transducer which measures the components of force in the plane of the surface on which the tire is moving and a strain gauge member which senses the component of force normal to the plane of the surface on which the tire is rolling. The use of the ball spline suspension made it necessary to mount the biaxial force transducer in a plane parallel to the surface of the road on which the tire being tested is rolling. The use of a biaxial force transducer to measure force components in a plane parallel to the road surface on which the tire is being tested and the use of a vertical strain gauge member to measure the force applied to the tire normal to the plane of the road surface requires a massive unsprung structure which results in tire load fluctuation due to the inertial loads imposed by this structure. Additionally, mounting of two independent force transducers presents severe spatial location problems in the wheel assembly area.